Master Gardeners - Mild winter can be destructive for plants

By Charlotte GlenPender County Cooperative Extension

Friday

Feb 3, 2012 at 7:33 AM

We are experiencing milder winters in southeastern North Carolina.

While it comes as no surprise to gardeners, the U.S. Department of Agriculture has made it official – we are experiencing milder winters in southeastern North Carolina. This recognition was formalized on Jan. 25 with the release of the updated plant hardiness zone map. Milder winters probably sound like good news for plants and gardeners, but in reality they can create more problems than opportunities in our region.

The plant hardiness zone map divides the country into numbered zones according to the average minimum winter temperatures that can be expected, based on 30 years of data collected between 1976 and 2005. It has long been used by gardeners to guide plant selection. In fact, most plants now come labeled as to which zones they are winter hardy. One thing to keep in mind about the zones is that they are based on average minimums, not extremes. During true extreme events such as the Christmas snow of 1989, which brought lows near 0 degrees to our area, even plants rated as hardy to our zone will be damaged or killed.According to the new map, all gardeners in the Cape Fear region are now securely located in zone 8a, and can expect average winter minimum temperatures to range from 10 to 15 degrees Fahrenheit. Under the old map, half of our area was categorized as zone 7b, with winter minimums between 5 to 10 degrees. While a five-degree increase may not sound like much of a change to warm-blooded mammals, it can be the difference between life and death for many plants.

In reality, minimum winter temperatures have never been the major challenge facing gardeners in the southeast. Much more damaging are the roller coaster fluctuations between mild and cold temperatures that characterize our winters. Plants that experience a rapid change from mild to cold are much more likely to be damaged than those that stay consistently cold over a long time. Plants are more prone to cold injury in the latter half of winter than in the beginning. This is because plants are able to measure the amount of cold weather to which they have been exposed. Each variety of plant has a specific amount of cold weather it must experience before it is ready to start growing. Once that level is reached, mild temperatures will trigger new growth to start. By late winter many common landscape plants, turf grasses and fruit trees grown in our region have reached their cold requirement. Increasing daylight hours along with a few warm days in February or March will tempt them to open their blossom and leaf buds. Once these buds are open they are much more susceptible to cold injury than when they were fully closed. When frost returns, these plants are caught out – and the buds that have opened are often killed. For fruit trees like plums and peaches, this can mean no fruit this year. In lawns, the result is often large dead spots that fail to turn green in spring. Centipede grass is particularly prone to this type of late winter cold injury. For flowers and shrubs, fluctuating temperatures can kill flower buds and damage new growth. Extreme events, such as the Easter freeze of 2007, can result in serious and lasting damage. Fluctuating winter temperatures often cause winter vegetables to bolt, or go to flower early. Onions are particularly prone to bolting in winter warm spells. This is bad news because once onions start to make a flower their bulbs will not gain any more size, even if you cut the flower off.

The particularly mild winter we are experiencing this year could be setting our lawns and gardens up for cold injury as we move closer to spring. There is little gardeners can do to hold plants back from the temptation to grow or protect them from sudden drops in temperature. One practice that can help is to avoid fertilizing too early. Do not apply any fertilizer to lawns until mid-April. While it is recommended to start fertilizing trees and shrubs in March, use a slow-release formulation such as Osmocote, or an organic fertilizer, rather than soluble fertilizers such as 10-10-10, which release all their nitrogen at once. Likewise in vegetable gardens, use slow-release fertilizers or make multiple light applications of soluble products rather than dumping a whole season's worth of nutrients on plants all at one time. Learn moreThe new plant hardiness zone map can be viewed online at PlantHardiness.ars.USDA.gov. If you have gardening questions contact your local cooperative extension office. In Pender County call 259-1235 8 a.m.-5 p.m. Monday-Friday, or visit us online at Pender.ces.NCSU.edu. Visit the Pender Gardener blog at PenderGardener.blogspot.com.

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